Download NGC 6231: a young open cluster under X-rays

NGC 6231: a young open
cluster under X-rays*
H. Sana
&
E. Gosset
Y. Naze
G. Rauw
J.-M. Vreux
&
H. Sung
* an adaptation of the novel ‘An XMM-Newton view of the young open cluster NGC 6231’
Contents

Generalities



XMM-Newton
NGC 6231
The X-ray emitters in NGC 6231 :



20-Dec-2006
O-type stars
B-type stars
Optically faint X-ray emitters
NGC 6231: a cluster under X-rays
XMM-Newton
• ESA X-ray Multi-Mirror observatory:
 December 1999
• 3 X-ray telescopes containing 58
concentric mirrors each
• large effective area
• high sensitivity
• high spectral resolution
• moderate spatial resolution
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NGC 6231: a cluster under X-rays
3
XMM-Newton
• ESA X-ray Multi-Mirror observatory:
 December 1999
• 3 X-ray telescopes containing 58
X-ray
spectro-photometry
:
concentric
mirrors
each
• largeeffective
energy area
• high
sensitivity
time of arrival
• high spectralofresolution
every single X-ray photon
• moderate spatial resolution
RGS: Reflection Grating Spectrometer (2 instr., Δλ= 0.04 Å )
EPIC: European Photon Imaging Camera (2 MOS + 1pn)
OM: Optical Monitor
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NGC 6231: a cluster under X-rays
4
The young open cluster NGC 6231
• α = 16h54m, δ = -41°50’ (J2000)
Scorpius
NGC 6231
N
E
2’
• d ~ 1.6 kpc
Sco OB1
• age ~ 3-5 Myr
NGC 6231
~5°
20-Dec-2006
N
E
• rich early-type star population
• core of the Sco OB1 association
NGC 6231: a cluster under X-rays
The X-ray observing campaign
• September 2001 : XMM-Newton
– 180 ks (~50h)
– splitted in six 30ks observations spread over 5 days
• One of the deepest exposures towards a young open cluster
RGS: Reflection Grating Spectrometer  not usable (too faint)
EPIC: European Photon Imaging Camera
OM: Optical Monitor  not usable (too bright)
20-Dec-2006
NGC 6231: a cluster under X-rays
The X-ray observing campaign
• September 2001 : XMM-Newton
– 180 ks (~50h)
– splitted in six 30ks observations spread over 5 days
• One of the deepest exposures towards a young open cluster
 ~ 30’
~ 108 early-type stars
* 92 B stars
* 15 O stars
* 1 WR
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NGC 6231: a cluster under X-rays
An XMM-Newton view of NGC 6231

610 X-ray sources

> 520 optical/IR counterparts

Contamination by foreground/background sources ?


~ 20 field stars (mostly F and G dwarfs)

~ 21 active stars

~ 16 extragalactic sources
Nature of the X-ray emitters in NGC 6231

O-type stars

B-type stars

Optically faint X-ray emitter
Sana et al. 2006a,
20-Dec-2006 A&A 454, 1047
NGC 6231: a cluster under X-rays
0.5-1.0 keV
1.0-2.5 keV
2.5-10.0 keV
O-type stars
are hot, massive stars
O-type stars:
– 30 kK < Teff < 45 kK
– 15 M < M* < ?
– 5×104 L < L* < 5×105 L
 short life-time (a few 106 yr)
 few in number
•
Powerful stellar winds
– 10-7 M yr-1 < Ṁ* < 10-4 M yr-1  affect the star’s evolution
– v∞ ~ 2000-3000 km s-1 ( v = v∞ (1–R*/r)
• Strong influence (radiative & kinetic energy, chemical input)
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NGC 6231: a cluster under X-rays
10
O-type stars
are hot, massive stars
O-type stars:
– 30 kK < Teff < 45 kK
– 15 M < M* < ?
– 5×104 L < L* < 5×105 L
 short life-time (a few 106 yr)
 few in number
A (very) few hot topics:
• Upper mass limit : ~ 100-150 M
• Formation process : coalescence vs. accretion
• Convective core but radiative enveloppe  no surface B field
still a few O stars have one ! (e.g. Gagne et al. 2005)
 fossile field ? / transported from the core to the enveloppe ?
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NGC 6231: a cluster under X-rays
11
X-ray emission from O-type stars
Rosat All Sky Survey
1978: launch of EINSTEIN:
 massive stars are
‘intrinsic’ X-ray emitters
Log LX
•
Berghöfer et al. 1997
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NGC 6231: a cluster under X-rays
Log Lbol
12
X-ray emission from O-type stars
Rosat All Sky Survey
1978: launch of EINSTEIN:
 massive stars are
‘intrinsic’ X-ray emitters
• Produced by shocks within
the dense layers of the winds
Log LX
•
• Strong but soft emission
– Lx ~ 1031-1033 erg s-1
– Emission peak ≤ 1keV
•
Berghöfer et al. 1997
Log Lbol
Harnden et al. 1979; Long &
White1980; Pallavicini et al. 1981; …
Canonical relation  Lx ~ 10-7 Lbol
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NGC 6231: a cluster under X-rays
13
X-ray emission from O-type stars
• Produced by shocks within
the dense layers of the winds
• Strong but soft emission
– Lx ~ 1031-1033 erg s-1
– Emission peak ≤ 1keV
•
Harnden et al. 1979; …
Canonical relation:
 Lx ~ 10-7 Lbol
Mass of star A larger than mass of star B
• Pollock 1987: WR binaries tend to be more X-ray luminous
compared to equivalent single stars
• Chlebowski & Garmany 1991: same trend for O-type stars
 signature of a wind interaction
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NGC 6231: a cluster under X-rays
14
O-type X-ray emitters in NGC 6231
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NGC 6231: a cluster under X-rays
16
O-type X-ray emitters in NGC 6231
All the O-type stars are detected
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17
O-type X-ray emitters in NGC 6231
All the O-type stars are detected
as
bright and soft X-ray sources
To be fitted by optically thin thermal plasma
(e.g. mekal models)  flux  Lx
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NGC 6231: a cluster under X-rays
18
O-type X-ray emitters in NGC 6231
Stars of spectral type O9 or earlier
To be fitted by optically thin thermal plasma
(e.g. mekal models)  flux  Lx
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NGC 6231: a cluster under X-rays
19
O-type X-ray emitters in NGC 6231
Filled
O-type
HD: 152248
Open : B-type
Sana et al. 2006b,
MNRAS
(cnt
s-1)372, 661
I
III
V
Binary
Single
LPVs
Sana et al. 2004,
MNRAS 350,809
CPD-41°7742 (10-3 cnt s-1)
Stars of spectral type O9 or earlier
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NGC 6231: a cluster under X-rays
Sana et al. 2005a,
A&A 441, 213
O-type X-ray emitters in NGC 6231
Filled : O-type
Open : B-type
Sana et al. 2006b,
MNRAS 372, 661
I
III
V
Binary
Single
LPVs
Canonical relation for O stars:
log LX  log Lbol  6.912 ( 0.153)
• dispersion ~ 40%  much more limited than previously admitted
• within our sample, O-type star X-ray variability only seen for probable CWBs
+ only identified mechanism that produces a significant deviation from the
‘canonical’ relation
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NGC 6231: a cluster under X-rays
O-type X-ray emitters in NGC 6231
What have we learned from XMM-Newton ?
I
• The canonical relation is tightly constrained
V
but still not understood on firm theoretical
ground !
• Our sample is inhomogeneous in terms of spectral sub-type
and luminosity class  same mechanism for all
• Confirmed by a recent study by Anthokin et al. on Cyg OB2
• Very limited room for the B field
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NGC 6231: a cluster under X-rays
B-type stars
– 10 kK < Teff < 30 kK
– 3 M < M* < 15 M
Berghöfer et al. 1997
• radiative enveloppe  no dynamo  no surface B field
• Weak winds  weak shocks (if any)
 No X-ray emission expected
but still … some B-type stars are associated to X-ray sources
e.g. Berghofer et al. 1997 : 10% detection rate
 hidden companion ? (e.g. Stelzer et al. 2003)
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NGC 6231: a cluster under X-rays
23
B-type X-ray emitters in NGC 6231
• 92 B-type stars in the FOV
• ~ 20% are associated with
an X-ray source
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24
B-type X-ray emitters in NGC 6231
B star
• 92 B-type stars in the FOV
• ~ 20% are associated with
an X-ray source
• fainter and harder than O stars
O star
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B-type X-ray emitters in NGC 6231
B star
• 92 B-type stars in the FOV
• ~ 20% are associated with
an X-ray source
• fainter and harder than O stars
X-ray faint star
• 4/15 display flare-like activity
• spectral properties
indistinguishable from those of
other faint X-ray sources
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NGC 6231: a cluster under X-rays
26
B-type X-ray emitters in NGC 6231
• 92 B-type stars in the FOV
• ~ 20% are associated with
an X-ray source
• fainter and harder than O stars
• 4/15 display flare-like activity
• spectral properties
indistinguishable from those of
other faint X-ray sources
 favours the hidden companion hypothesis
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NGC 6231: a cluster under X-rays
27
B-type X-ray emitters in NGC 6231
• 92 B-type stars in the FOV
• ~ 20% are associated with
an X-ray source
• fainter and harder than O stars
• 4/15 display flare-like activity
• spectral properties
indistinguishable from those of
other faint X-ray sources
 favours the hidden companion hypothesis
20-Dec-2006
NGC 6231: a cluster under X-rays
but still…
28
B-type X-ray emitters in NGC 6231
Filled : O-type
Open : B-type
Sana et al. 2006b,
MNRAS 372, 661
I
III
V
Binary
Single
LPVs
LX-Lbol relation for detected B-stars:
log LX  0.22(0.06) log Lbol  22.8 ( 2.4)
• limited dispersion; linear correlation coefficient r ~ 0.75
significant at 0.99 confidence level
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NGC 6231: a cluster under X-rays
B-type X-ray emitters in NGC 6231
What have we learned from XMM-Newton ?
I
• The detected X-ray sources associated with B-type stars are
V
probably ‘hidden’ companion
• There seems to be a link between Lx and Lbol
 if real, its origin is ????
20-Dec-2006
NGC 6231: a cluster under X-rays
Optically faint X-ray
sources in NGC 6231

610 X-ray sources
~ 60 foreground/background sources
~ 15 O-type stars + 1 WR
~ 18 B-type stars (if any)
 What

•
20-Dec-2006
are the > 500 remaining sources ?
most of them have V~16-20 optical counterparts
NGC 6231 age ~ 3-5 Myr  All stars with M
< 2 Msol are still in the PMS stage
NGC 6231: a cluster under X-rays
31
Pre-Main Sequence (PMS) stars
The main kind of PMS are the T Tauri objects :
•
Classical T Tauri (cTTs) :
• emission in the Balmer lines
• near-IR excess (accretion disk)
• Weak-line T-Tauri (wTTs) :
• no significant IR excess
• weak Balmer emission (if any)
 difficult to disentangle from field stars
 X-ray is one of the best criterion
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An XMM-Newton view of NGC 6231
7.0M
0.2M
X-ray source
X-ray + Hα candidate
X-ray + Hα emitter
20-Dec-2006Sana
et al. 2007, MNRAS, submitted
NGC 6231: a cluster under X-rays
0.5-1.0 keV
1.0-2.5 keV
2.5-10.0 keV
An EPIC view of NGC 6231
7.0M
0.2M
X-ray source
X-ray + Hα candidate
X-ray + Hα emitter
20-Dec-2006Sana
0.5Myr
1.5Myr
4.0Myr
10.0Myr
20.0Myr
et al. 2007, MNRAS, submitted
NGC 6231: a cluster under X-rays
0.5-1.0 keV
1.0-2.5 keV
2.5-10.0 keV
2Myr
An EPIC view of NGC 6231
O-type stars
8Myr
20-Dec-2006Sana
et al. 2007, MNRAS, submitted
NGC 6231: a cluster under X-rays
0.5-1.0 keV
1.0-2.5 keV
2.5-10.0 keV
PMS X-ray emitters in NGC 6231
What have we learned from XMM-Newton ?
I
• XMM has allowed to disentangle the PMS from the
V
numerous field stars
• There is no spatial trends between the cTTs and the wTTs
• There is no very significant difference in their ages
• Star formation in NGC 6231 was most probably not a
punctual event
• Formation scenario is compatible with the coalescence
model for massive star formation
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NGC 6231: a cluster under X-rays
What have we learned from XMMNewton

One of the deepest X-ray observations of a young open cluster,
 spread over 5 days
 Crowded FOV: 610 sources (mostly PMS)
 star formation history in NGC 6231: not a punctual event

O-type stars display a limited dispersion around the canonical relation :
 within our sample, X-ray variability only seen for probable CWBs
 only identified mechanism that produces a significant deviation from the
‘canonical’ relation



we need to compare the new ‘canonical’ relation with other relations derived
in a homogeneous way
At least two exceptional O-type colliding wind binaries (CWBs)
X-ray emission associated with B-type stars?  PMS companions
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NGC 6231: a cluster under X-rays
Final remarks

X-ray observations supported by high quality optical data
are a powerful tool across the HR diagram

Young open clusters are privileged laboratories
-
homogeneous population : age, distance, environment
Thank you for your attention
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NGC 6231: a cluster under X-rays
Colliding wind binaries (CWBs)
• In a massive binary system
– The winds from the two stars collide
 Gas heated up:T ≈ a few 107 K
kT ≈ a few keV
 substantial X-ray emission
 can display phase-locked modulations due :
• to the changing opacities along the line of sight
• in an eccentric system, to the variations of the shock strength
because of the changing separation
– The geometry of the interaction depends on the balance
between the ram pressure of the two winds
 the interaction region wraps around the star that has the
weaker wind
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NGC 6231: a cluster under X-rays
Traquer les propriétés physiques : la
spectroscopie, un outil puissant
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Traquer les propriétés physiques
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41
O miroir mon beau miroir, quelle est la
plus massive d’entre nous ?

Contraintes observationnelles

Amas des Arches
Centre galactique
Amas des Arches :
* Pop.: 2000 étoiles
* Age ~ 2-2.5 Myr
* D ~ 25 000 a.l.
Particules de
haute énergie
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43
O miroir mon beau miroir, quelle est la
plus massive d’entre nous ?

Contraintes observationnelles


Considération théorique


Déficit d’étoiles au dessus de 120-150 M
Ledoux : instabilité vibrationnelle
Mesures directes : difficiles à mettre en oeuvre


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R136
WR20a
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Emission X des étoiles O

Produite par des chocs
dans les basses couches
du vent

Emission X intense mais
relativement peu
énergétique
 Lx ~ 1031-1033 erg s-1
 Pic d’émission ≤ 1keV

Relation canonique* :
 Lx ~ 10-7
Berghöfer et al. 1997
Lbol
* large dispersion observée
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NGC 6231: a cluster under X-rays
45
Binaires à collision de vents
7 2
T
1
.4

10
v
K
8
,
2
k
T
1
.2
v
keV
8
,
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46
Binaires à collision de vents
courbes de lumière X
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Interaction d’un
type particulier
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Conclusions :
* Cent mille à un million de fois plus
lumineuses que le soleil
* Au moins 5 à 6 fois plus chaudes,
 objets fascinants et variés
* Leur leitmotiv : "Vivre vite, mourir jeune …
…et faire un maximum de bruit
Merci de votre attention
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